1. Field of the Invention
The present invention relates to a switching control apparatus configured to control a switch for supplying or cutting off electric power from a commercial alternating current power supply to a predetermined load, and to an image forming apparatus using the switching control apparatus.
2. Description of the Related Art
A thyristor and a gate-controlled semiconductor switch (generally known as TRIAC (trademark) and hereinafter simply referred to as TRIAC) are known as a switch for supplying or cutting off electric power from a commercial alternating current power supply to a predetermined load. A switching control apparatus for controlling the thyristor or the TRIAC is driven by a control-purpose power supply, and applies or cuts off a gate current supplied from the commercial alternating current power supply to a gate terminal of the thyristor or the TRIAC, to thereby perform switching operation of the thyristor or the TRIAC.
For instance, Japanese Patent Application Laid-Open No. 2009-207241 proposes a thyristor gate drive circuit, in which a photocoupler is connected in parallel to the thyristor, and the thyristor and the photocoupler are controlled to be turned on and off alternately to each other. Thus, even when the control-purpose power supply does not work correctly, electric power can be supplied to a load connected to the commercial alternating current power supply.
However, in the related art described above, although the thyristor can be turned on even when electric power cannot be supplied from the control-purpose power supply to the thyristor gate drive circuit, lower power consumption that is required in recent years is not taken into consideration. It is because two photocouplers need to be turned on for maintaining the OFF state of the thyristor. In addition, a leakage current flowing from the commercial alternating current power supply to the load through the thyristor gate drive circuit when the thyristor is turned off cannot be neglected in order to achieve low power consumption.